For the development of a seawater RO desalination system utilizing the high static pressure of the deep-sea, an experimental study on the desalination of seawater was actually performed in the East China Sea at 300-600m below the surface of the sea, using a desalination device in which a tube type membrane with a cylindrical rod put into the central axis of the membrane tube was installed as reverse osmosis membrane.It has been proved that fresh water can be obtained easily from seawater by this system without the use of electrical power.Also a numerical analysis was performed to determine the axial variation of velocity, concentration and pressure in laminar natural convection flow, caused by concentration difference in the moderately narrow annular passage formed between the tube type membrane and the cylindrical rod. A dimensionless equation was formulated from the numerical results to predict the transmitted fresh water flow rate from the width and the length of the passage, the submerged depth of device, the permeability of membrane and physical properties of seawater. The transmitted fresh water flow rate exhibited an increase with decreasing width of passage, reached a maximum at the width of about 1-2 mn, followed by a rapid decrease, and thus the existence of the optimum width of passage was realized.The experimental data showed good agreement with those calculated from the dimensionless equation and the essential information for optimizing the design of a desalination device were presented.